Focusing problems with my ED100

[Chris M48]

I can’t seems to get good focus with my SW ED100 Refractor.

Below is an example of a typical image taken with my T3 (1100D).

It’s an APO doublet so I’m very wary about disassembly.

Any suggestions please?

 

Edited June 18, 2019 by Chris M48
Adding descriptive image

[MARS1960]

Focus looks ok to me, have you tried using a bhatinov mask?

[FaDG]

Is it 100% crop? I don't find it that bad at all.

What are the image specs (exposure, guide, processing)? what was the seeing like that evening? Is your camera monochrome modded?

[DaveS]

Focus looks pretty good to me too, perhaps just a *tad* soft.

As the others said, have you tried using a Bahtinov Mask?

[Chris M48]

No cropping at all, minimal processing with just DSS but yes the camera has had its filters removed. I use a Bhatinov mask on all scopes as a matter of habit.

A dozen subs with darks,flats and bias. 120 secs at 800iso.

Can’t seem to get small stars as pinpoint as once was the case.

 

 

[michael8554]

You can only properly judge focus on one short not saturated sub of a bright star.

Long exposures will have guide errors, and stacking then gives you round but fatter stars.

Michael

[DaveS]

WHS

If you've used a Bahtinov Mask to nail focus then I'd be looking at other sources of "fatness", especially guiding errors, though TBH the stars look pretty sharp to me.

If the stars really are fatter than they were, then mount issues would be my first point of attack. And don't dismantle the lens, it's unlikely to be the problem.

[FaDG]

On 18/06/2019 at 20:30, Chris M48 said:

No cropping at all, minimal processing with just DSS but yes the camera has had its filters removed. I use a Bhatinov mask on all scopes as a matter of habit.

A dozen subs with darks,flats and bias. 120 secs at 800iso.

Can’t seem to get small stars as pinpoint as once was the case.

 

 

Could you please send a 100% crop at the centre of a SINGLE unprocessed sub?

My images improved seriously when passing from the CG5 to the HEQ5, and that was with the ED80 @ 600mm, not the 100! Now it's fine even with the 150pds.

What were the guiding stats for that session? --> you're imaging at 1.19arcsec/pixel, need the guiding to stay well below 0.8 RMS, otherwise what you see is bloating due to the guiding/seeing.

 

 

[Chris M48]

That’s great stuff folks, plenty to work on there.

Thanks everyone.

[alan4908]

On 18/06/2019 at 16:40, Chris M48 said:

I can’t seems to get good focus with my SW ED100 Refractor.

Below is an example of a typical image taken with my T3 (1100D).

It’s an APO doublet so I’m very wary about disassembly.

Any suggestions please?

Hi Chris

I suggest you download a free trial of CCDInspector (https://www.ccdware.com/products/ccdinspector/) which will enable you to quantitatively evaluate your focus, camera tilt and curvature.  In order to do this accurately you need to put it in averaging mode (say 5 images) and feed it calibrated but unstacked subs. Whilst you can use it on processed images, it can give very misleading results.  Having said all that...... if I put your image through CCDInspector it does indicate that you have quite a bit of camera tilt (12%) and your star aspect ratio is 20 which isn't great.  If you put the image through PI's aberration inspector you can clearly see the effect on the stars at different points of the image see below (top left/bottom left/center/top right/top left).

 

Poor aspect star ratios can be created by poor guiding and camera tilt can be caused by focuser droop. So, to reduce any errors created by your mount and reduce focuser droop, I'd suggest pointing your scope vertically and taking 5 very short exposures (eg 10s) and then feeding these individual subs through CCDInspector  and examining the result. By using this methodology you should be able to decouple focusing issues from other effects. 

Alan

[ollypenrice]

Your pixel scale is 1.19"PP which is fine enough to very susceptible indeed to seeing. On a good stable night you should be able to image at that resolution but bad seeing could have you struggling to catch half that level of detail.

The B mask is a great aid to focusing but it doesn't give you any information about the seeing. If you also use Full WIdth Half Max (available in most astro camera control software) you'll be able to see the smallest stellar radius which can be achieved on that night.  Once you've used it regularly you'll get to know what is good, what is OK and what is abominable! Because I use mono cameras I can shoot only colour on bad nights and wait for good ones to shoot luminance which will put the detail back in the image. You could, perhaps, do likewise by classifying your captures according to FWHM. Put the best ones into a separate stack and process them for sharpness and small stars. Call that synthetic luminance. Then process the lot for colour and apply the synthetic luminance over the top as luminosity.

Olly

[Chris M48]

Thanks again everyone. Never disappointed on this forum.

Took the scope to www.intrasights.co.uk and met Harry Truman the proprietor who is best known for his binocular repairs but occasionally works on telescopes.

I had previously cleaned the lenses but obviously not to such a professional standard as with Harry. He put quite a few hours in over a 10 day period but only charged me £70 which I found more than reasonable.

Judge for yourselves the result.

I should have given the camera a clean too. What am I like?